The present disclosure relates to a centralizing assembly for connecting segments of sucker rods together and to space the connected sucker rod sections away from the sides of the well.
Once a well has been established for the production of fluid hydrocarbons it is often necessary to pump the fluid out of the well when the fluid has a high viscosity or a relatively low pressure. Most artificial lift wells in the U.S. and Canada consist of a downhole pump attached to a surface power source by series of connected sucker rods. Each sucker rod is machined from a solid rod approximately 25 to 30 feet long with a central diameter between ⅝″ and 1⅛″ with a threaded pin and shoulder at either end of the rod. A “sucker rod coupling” is a cylindrically internally threaded member and is used to connect segments of sucker rod together to create an assembly called a “sucker rod string”. The sucker rod couplings are typically 4″ long and have a larger outer diameter than the raised shoulders of the sucker rod.
The sucker rod string is fed through a concentric tubing string consisting of 30 foot sections of tubing which are threaded together and typically have a nominal inside diameter between 2⅜″ and 4½″. The function of the sucker rod string is to actuate the downhole pump to force fluid to the surface by pumping the fluid flow through the annulus formed by the sucker rod string and the tubing string. Progressing cavity pumps, will rotate the sucker rod string and reciprocating pumps will move the sucker rod string up and down the axial direction of the sucker rod string.
The present disclosure was developed for applications with rotating sucker rod strings. It will be discussed herein in connection with the problems associated with rotating sucker rod strings, particularly those applications which exert high torques on the sucker rod string downhole. However, the product can be used in reciprocating pumping also.
When sucker rod string is rotated within a well that deviates from vertical, the string tends to lie on the one side of the tubing and the rod string rotates eccentrically about this point. This eccentric motion allows the steel sucker rod couplings, which have a larger outer diameter than the sucker rod, to slap and grind against the steel tubing causing wear and severe damage to the tubing wall. The resulting tubing wall failure is disastrous for the well operation and requires expensive repairs. It has become industry practice to centralize the rod string within the tubing with a soft non-metallic material to prevent steel-on-steel contact between the couplings and the tubing wall. This soft non-metallic centralizer, or guide, can be mounted on the rod string in a variety of methods. However, mounting the centralizer at the connecting point of a pair of sucker rods ensures that the large diameter couplings are prevented from contacting the tubing wall. If the centralizer were mounted on the narrow sucker rod stem in the center of the sucker rod the couplings might still contact the tubing wall.
The industry has recently been experimenting with high strength materials and alternative manufacturing techniques in the production of sucker rods resulting in the ability to apply higher amounts of torque to the rod string downhole. However, the same couplings and guides are being used to connect the rod string. In order for the centralizer mandrel to transmit the higher torque without material failure a greater bearing contact surface between the mandrel and the couplings was required. However, in a standard one-piece centralizer design as the bearing contact surface increases, the shoulder area formed by the interface of the mandrel and the couplings which fixes the soft non-metallic centralizer rotor decreases and results in premature failure, axial travel of the rotor and severe tubing wall damage.
U.S. Pat. No. 4,919,202 issued to Clintberg discloses a sucker rod guide bearing having a free spinning vaned rotor of soft resilient material which is fixed by two large diameter washers and a mandrel which are attached to sucker rod couplings. However, the bearing surface formed between the mandrel and the free spinning washer does maximize the contact area given that the diameter of the mandrel is less than that of the coupling. Consequently, the sucker rod guide bearing is not suitable for high torque applications. The metallic washer with a larger diameter than the coupling also introduces the likelihood of metal-metal contact with the tubing wall once the rotor has been worn down. While the resulting metal-metal wear is generally less damaging, given the washer is softer than the tubing wall, the large diameter washer is undesirable as the large diameter and sharp edge are have the possibility to leave severe localized wear on the tubing.